JP6749893B2 - User terminal and mobile communication method - Google Patents

Description

The present invention relates to a user terminal and a mobile communication method for selecting a selection target cell used as a serving cell from a plurality of cells.

In 3GPP (3rd Generation Partnership Project), which is a standardization project for mobile communication systems, a technique for selecting a selection target cell to be used as a serving cell from a plurality of cells operated at different frequencies has been proposed (for example, Non-Patent Document 1). 1).

Specifically, when the start condition is satisfied, the user terminal measures the quality of an adjacent cell adjacent to the current serving cell and selects a selection target cell to be used as a serving cell from cells that satisfy the selection condition.

3GPP Technical Report “TS36.304 V12.4.0” March 2015

A first feature is a user terminal, which suspends the random access procedure according to a predetermined message received from a connection target cell in a random access procedure, and selects a cell to be used as a serving cell from a plurality of cells. The gist is to provide a control unit for selecting.

A second feature is a mobile communication method, wherein the random access procedure is interrupted in response to a predetermined message received from a connection target cell in the random access procedure, and a selection target to be used as a serving cell is selected from a plurality of cells. The gist is to include the step of selecting cells.

FIG. 1 is a configuration diagram of an LTE system according to an embodiment. FIG. 2 is a block diagram of the UE 100 according to the embodiment. FIG. 3 is a block diagram of the eNB 200 according to the embodiment. FIG. 4 is a protocol stack diagram of the wireless interface according to the embodiment. FIG. 5 is a configuration diagram of a radio frame used in the LTE system according to the embodiment. FIG. 6 is a diagram for explaining an application scene according to the embodiment. FIG. 7 is a sequence diagram showing a mobile communication method according to the embodiment. FIG. 8 is a sequence diagram showing a mobile communication method according to the second modification. FIG. 9 is a sequence diagram showing a mobile communication method according to the second modification. FIG. 10 is a sequence diagram showing a mobile communication method according to the third modification.

In the mobile communication system described in the background art, a user terminal in an RRC idle state that uses a certain cell as a serving cell may transition to the RRC connected state using a certain cell as a connection target cell. That is, the number of user terminals in the RRC idle state that use a certain cell as a serving cell is a potential load for the certain cell.

However, the load of each cell is not considered in the selection of the selection target cell. Therefore, there is a possibility that the load distribution of each cell cannot be properly performed.

Therefore, the embodiment has been made to solve the above-described problems, and provides a user terminal and a mobile communication method that enable appropriate load distribution of each cell.

The mobile communication system according to the embodiment will be described below with reference to the drawings. In the following description of the drawings, the same or similar parts are designated by the same or similar reference numerals.

However, it should be noted that the drawings are schematic and the ratio of each dimension may be different from the actual one. Therefore, specific dimensions should be determined in consideration of the following description. Further, it is needless to say that the drawings include portions in which dimensional relationships and ratios are different from each other.

[Summary of Disclosure]
A user terminal according to the outline of the disclosure interrupts the random access procedure in response to a predetermined message received from a connection target cell in a random access procedure, and selects a selection target cell to be used as a serving cell from a plurality of cells. A control unit is provided.

In the user terminal according to the outline of the disclosure, the control unit interrupts the random access procedure and selects the selection target cell in response to a predetermined message received from the connection target cell in the random access procedure. That is, when the load of the connection target cell is high, the user terminal is promptly prompted to select the selection target cell. Therefore, the load distribution of each cell can be appropriately performed. In other words, the user terminal in the RRC idle state can be appropriately distributed to each cell.

[Embodiment]
An embodiment will be described below by taking an LTE system based on the 3GPP standard as an example of a mobile communication system.

(System configuration)
A system configuration of the LTE system according to the embodiment will be described. FIG. 1 is a configuration diagram of an LTE system according to an embodiment.

As illustrated in FIG. 1, the LTE system according to the embodiment includes a UE (User Equipment) 100, an E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) 10, and an EPC (Evolved Packet Core) 20.

The UE 100 corresponds to a user terminal. The UE 100 is a mobile communication device, and performs radio communication with a cell formed by the eNB 200 (a serving cell when the UE 100 is in the RRC connected state). The configuration of the UE 100 will be described later.

The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10 includes an eNB 200 (evolved Node-B). The eNB 200 corresponds to a radio base station. The eNBs 200 are connected to each other via the X2 interface. The configuration of the eNB 200 will be described later.

The eNB 200 forms one or a plurality of cells and performs wireless communication with the UE 100 that has established a connection with the own cell. The eNB 200 has a radio resource management (RRM) function, a user data routing function, a measurement control function for mobility control/scheduling, and the like. The “cell” is used as a term indicating a minimum unit of a wireless communication area, and is also used as a term indicating a function of performing wireless communication with the UE 100.

The EPC 20 corresponds to the core network. The EPC 20 includes an MME (Mobility Management Entity)/S-GW (Serving-Gateway) 300. The MME performs various mobility controls for the UE 100. The S-GW controls transfer of user data. The MME/S-GW 300 is connected to the eNB 200 via the S1 interface. The E-UTRAN 10 and the EPC 20 make up a network of the LTE system.

FIG. 2 is a block diagram of the UE 100. As illustrated in FIG. 2, the UE 100 includes a plurality of antennas 101, a wireless transceiver 110, a user interface 120, a GNSS (Global Navigation Satellite System) receiver 130, a battery 140, a memory 150, and a processor 160. The memory 150 and the processor 160 form a control unit. The wireless transceiver 110 and the processor 160 constitute a transmitter and a receiver. The UE 100 may not have the GNSS receiver 130. Further, the memory 150 may be integrated with the processor 160, and this set (that is, a chip set) may be a processor.

The antenna 101 and the wireless transceiver 110 are used for transmitting and receiving wireless signals. The wireless transceiver 110 converts a baseband signal (transmission signal) output by the processor 160 into a wireless signal and transmits the wireless signal from the antenna 101. The wireless transceiver 110 also converts a wireless signal received by the antenna 101 into a baseband signal (received signal) and outputs the baseband signal to the processor 160.

The user interface 120 is an interface with a user who has the UE 100, and includes, for example, a display, a microphone, a speaker, and various buttons. The user interface 120 receives an operation from the user and outputs a signal indicating the content of the received operation to the processor 160. The GNSS receiver 130 receives the GNSS signal in order to obtain position information indicating the geographical position of the UE 100, and outputs the received GNSS signal to the processor 160. The battery 140 stores the power to be supplied to each block of the UE 100.

The memory 150 stores programs executed by the processor 160 and information used for processing by the processor 160. The processor 160 includes a baseband processor that performs modulation/demodulation and encoding/decoding of a baseband signal, and a CPU (Central Processing Unit) that executes a program stored in the memory 150 to perform various processes. The processor 160 may further include a codec that encodes/decodes audio/video signals. The processor 160 executes various processes and various communication protocols described later.

FIG. 3 is a block diagram of the eNB 200. As shown in FIG. 3, the eNB 200 includes a plurality of antennas 201, a wireless transceiver 210, a network interface 220, a memory 230, and a processor 240. The memory 230 and the processor 240 form a control unit. The wireless transceiver 210 (and/or the network interface 220) and the processor 240 constitute a transmitter and a receiver. Further, the memory 230 may be integrated with the processor 240, and this set (that is, a chip set) may be a processor.

The antenna 201 and the wireless transceiver 210 are used for transmitting and receiving wireless signals. The wireless transceiver 210 converts the baseband signal (transmission signal) output by the processor 240 into a wireless signal and transmits the wireless signal from the antenna 201. The wireless transceiver 210 also converts a wireless signal received by the antenna 201 into a baseband signal (received signal) and outputs the baseband signal to the processor 240.

The network interface 220 is connected to the adjacent eNB 200 via the X2 interface and is connected to the MME/S-GW 300 via the S1 interface. The network interface 220 is used for communication performed on the X2 interface and communication performed on the S1 interface.

The memory 230 stores programs executed by the processor 240 and information used for processing by the processor 240. The processor 240 includes a baseband processor that performs modulation/demodulation and encoding/decoding of a baseband signal, and a CPU that executes a program stored in the memory 230 to perform various processes. The processor 240 executes various processes and various communication protocols described later.

FIG. 4 is a protocol stack diagram of a radio interface in the LTE system. As shown in FIG. 4, the radio interface protocol is divided into first to third layers of the OSI reference model, and the first layer is a physical (PHY) layer. The second layer includes a MAC (Medium Access Control) layer, an RLC (Radio Link Control) layer, and a PDCP (Packet Data Convergence Protocol) layer. The third layer includes an RRC (Radio Resource Control) layer.

The physical layer performs encoding/decoding, modulation/demodulation, antenna mapping/demapping, and resource mapping/demapping. User data and control information are transmitted via a physical channel between the physical layer of UE100 and the physical layer of eNB200.

The MAC layer performs data priority control, retransmission processing by hybrid ARQ (HARQ), random access procedure, and the like. User data and control information are transmitted via the transport channel between the MAC layer of the UE 100 and the MAC layer of the eNB 200. The MAC layer of the eNB 200 includes a scheduler that determines a transport format (transport block size, modulation/coding method (MCS)) of uplink and downlink and a resource block allocated to the UE 100.

The RLC layer uses the functions of the MAC layer and the physical layer to transmit data to the RLC layer on the receiving side. User data and control information are transmitted via a logical channel between the RLC layer of the UE 100 and the RLC layer of the eNB 200.

The PDCP layer performs header compression/decompression and encryption/decryption. Also, it should be noted that the PDCP layer is formed with a transmitting entity for transmitting a data unit (PDCP PDU) or a receiving entity for receiving a data unit (PDCP PDU).

The RRC layer is defined only in the control plane that handles control information. Control information (RRC message) for various settings is transmitted between the RRC layer of the UE 100 and the RRC layer of the eNB 200. The RRC layer controls logical channels, transport channels and physical channels according to establishment, re-establishment and release of radio bearers. When there is a connection (RRC connection) between the RRC of the UE 100 and the RRC of the eNB 200, the UE 100 is in the RRC connected state, and when there is no connection (RRC connection) between the RRC of the UE 100 and the RRC of the eNB 200. , UE 100 is in the RRC idle state.

A NAS (Non-Access Stratum) layer located above the RRC layer performs session management and mobility management.

FIG. 5 is a configuration diagram of a radio frame used in the LTE system. In the LTE system, OFDMA (Orthogonal Frequency Division Multiplexing Access) is applied to the downlink, and SC-FDMA (Single Carrier Frequency Division Multiple Access) is applied to the uplink.

As shown in FIG. 5, the radio frame is composed of 10 subframes arranged in the time direction. Each subframe is composed of two slots arranged in the time direction. The length of each subframe is 1 ms and the length of each slot is 0.5 ms. Each subframe includes a plurality of resource blocks (RBs) in the frequency direction and a plurality of symbols in the time direction. Each resource block includes a plurality of subcarriers in the frequency direction. One resource element (RE) is configured by one symbol and one subcarrier. Further, among the radio resources (time/frequency resources) assigned to the UE 100, frequency resources can be specified by resource blocks, and time resources can be specified by subframes (or slots).

(Applicable scene)
The applicable scenes will be described below. FIG. 6 is a diagram for explaining an application scene according to the embodiment.

As shown in FIG. 6, it has several eNB200 (For example, eNB200#1, eNB200#2, eNB200#3, eNB200#4). The eNB 200#1 has a cell #1 as a wireless communication area, the eNB 200#2 has a cell #2 as a wireless communication area, and the eNB 200#3 has a cell #3-1 as a wireless communication area. And the cell #3-2, and the eNB 200#4 has the cell #4-1 and the cell #4-2 as a wireless communication area.

The cell #2, the cell #3-1, the cell #3-2, the cell #4-1 and the cell #4-2 overlap with the cell #1. In addition, each cell is operated at a frequency different from each other. A priority is set for the frequency at which each cell is operated. The correspondence relationship between the frequency and the priority is included in the system information (SIB: System Information Block) notified from the eNB 200.

Under such a premise, the UE 100 selects a selection target cell used as a serving cell from a plurality of cells operated in different frequencies. Specifically, when the start condition is satisfied, the UE 100 measures the quality of the adjacent cell adjacent to the current serving cell and selects the selection target cell to be used as the serving cell from the cells that satisfy the selection condition.

First, the starting conditions are as shown below.

(A1) Frequency having a higher priority than the frequency priority of the current serving cell-The UE 100 constantly measures the quality of the frequency having a high priority.

(A2) A frequency having a priority equal to or lower than the priority of the frequency of the current serving cell-The UE 100 has a frequency having an equal priority or a low priority when the quality of the current serving cell is below a predetermined threshold. To measure the quality of.

Secondly, the selection conditions are as shown below.

(B1) The priority of the frequency of the adjacent cell is higher than the priority of the current serving cell-The UE 100 is a cell that satisfies the relationship of _Squal >Thresh _{X, HighQ} over a predetermined period (Treletion _RAT ), or a predetermined period ( A cell that satisfies the relationship of Srxlev>Thresh _{X, HighP} over the Treselection _RAT ) is selected as a selection target cell. In such a case, the criterion that the adjacent cell should satisfy may be referred to as “S-criteria”.

However, _Squal represents the cell selection quality level, and is calculated by Squal=Q _qualmeas −(Q _qualmin +Q _{qualminoffset} )−Qoffset _temp . Q _Qualmeas is the quality level of the neighboring cell _{(RSRQ), Q qualmin} is the minimum required quality _{level, Q Qualminoffset} is a predetermined offset that is constantly applied to the adjacent cells, _{Qoffset temp} is adjacent cells Is an offset that is temporarily applied to. Thresh _{X, HighQ} are predetermined thresholds.

Further, Srxlev represents a cell selection reception level, and is calculated by Srxlev=Q _rxlevmeas- (Q _rxlevmin +Q _{rxlevminoffset} )-Pcompensation-Qoffset _temp . Q _rxlevmeas is a reception level (RSRP) of a neighbor cell, Q _rxlevmin is a minimum required reception level, Q _{rxlevminoffset} is a predetermined offset constantly applied to the neighbor cell, and P compensation is an uplink. Qoffset _temp , which is a parameter related to the capability, is an offset temporarily applied to a neighboring cell. Thresh _{X, HighP} are predetermined threshold _values .

(B2) The priority of the frequency of the neighboring cell is the same as the priority of the current serving cell-The UE 100 calculates the ranking R _s of the current serving cell and the ranking R _{n of the} neighboring cell, and at the same time, the predetermined period (Tresection _RAT ). A cell having a ranking R _n higher than R _s is selected as a selection target cell. In such a case, the criterion that the adjacent cell should satisfy may be referred to as “R-criteria”.

However, _{R s is} calculated by _{R s = Q meas, s +} Q Hyst -Qoffset temp. R _{n is,} R _n = _Q meas, is calculated by n _{-Qoffset-Qoffset temp.} Q _meas and _s are reception levels (RSRP) of the current serving cell, and Q _meas and _n are reception levels (RSRP) of neighboring cells. Q _Hyst is a hysteresis value for facilitating _reselection of the current serving cell as the selection target cell. Qoffset _temp is an offset temporarily applied to the current serving cell and neighboring cells.

(B3) The priority of the frequency of the adjacent cell is lower than the priority of the current serving cell-The _{UE 100} satisfies _Squal <Thresh _{Serving, LowQ} for a predetermined period (Treselection _RAT ) or a predetermined period (Treselection _RAT). ), Srxlev<Thresh _{Serving, LowP} is satisfied, and the selection target cell is selected from the adjacent cells by the same method as (B1) described above.

However, Thresh _{Serving, LowQ,} and Thresh _{Serving, LowP} are predetermined threshold _values like Thresh _{X, HighQ} and Thresh _{X, HighP} .

Note that various parameters used in the selection of the selection target cell are included in the broadcast information (SIB: System Information Block) broadcast from the eNB 200. Various parameters, priority frequency (cellReselectionPriority), a predetermined period (Treselection _RAT), various offset _{(Q qualminoffset, Q rxlevminoffset, Qoffset} temp, Q Hyst, Qoffset), various threshold _{(Thresh X, HighQ, Thresh X} , HighP, Thresh _{Serving, LowQ} , Thresh _{Serving, LowP} ).

In the embodiment, the UE 100 (control unit) interrupts the random access procedure according to a predetermined message received from the connection target cell in the random access procedure, and selects a selection target cell to be used as a serving cell from a plurality of cells. To do. The predetermined message is a message transmitted from the connection target cell to the UE 100 in the random access procedure, and may be, for example, a Random Access Response or a Contention Resolution. Further, it is preferable that the predetermined message is transmitted from the connection target cell when the load of the connection target cell is equal to or higher than a certain level.

Here, the UE 100 (control unit) lowers the priority of selecting the connection target cell as the selection target cell in response to the reception of the predetermined message. For example, the UE 100 (control unit) changes the priority (cellReselectionPriority) of the connection target cell to the lowest priority (lowest priority). Alternatively, the UE 100 (control unit) may lower the frequency priority (cellReselectionPriority) of the connection target cell by a predetermined number of steps.

In the embodiment, the predetermined message may include a parameter used in the selection of the selection target cell. Parameters used in the selection of the selected target cell, the priority of the frequency (cellReselectionPriority), a predetermined period (Treselection _RAT), various offset _{(Q qualminoffset, Q rxlevminoffset, Qoffset} temp, Q Hyst, Qoffset), various threshold values _{(Thresh X, HighQ} , Thresh _{X, HighP} , Thresh _{Serving, LowQ} , Thresh _{Serving, LowP} ). In such a case, it is preferable that the parameter used for selecting the selection target cell is a parameter that makes it difficult for the connection target cell to be selected as the selection target cell. The UE 100 (control unit) selects a selection target cell from a plurality of cells based on the above-mentioned parameters.

Alternatively, the predetermined message may include recommendation information recommending that a cell other than the connection target cell be selected as the selection target cell. The UE 100 (control unit) selects the selection target cell used as the serving cell from the adjacent cells other than the connection target cell in response to the reception of the recommendation information.

The recommendation information may include a list of candidate cells that can be selected as the selection target cell. The UE 100 (control unit) selects the selection target cell to be used as the serving cell from the adjacent cells included in the candidate cell list in response to the reception of the recommendation information.

Furthermore, the recommendation information preferably includes an additional offset to be applied to a metric for determining whether or not the candidate cell should be selected as the selection target cell so that the candidate cell is likely to be selected as the selection target cell. The metric is a value such as _Squal (Q _qualmeas =RSRQ), Srxlev, (Q _rxlevmeas =RSRP), or the ranking R _n described above. The additional offset is, for example, a value for changing these values so that the candidate cell is easily selected as the selection target cell.

In such a case, the UE 100 (control unit) should select the candidate cell as the selection target cell as the metric for determining whether or not the connection target cell should be selected as the selection target cell when the additional offset is applied. If it is below the metric for determining whether or not the selection target cell is selected, it is preferable to start selection of the selection target cell. On the other hand, when applying the additional offset, the UE 100 (control unit) determines whether the metric for determining whether or not the connection target cell should be selected as the selection target cell is the candidate cell as the selection target cell. If it exceeds the metric for determining, the random access procedure is preferably restarted.

(Mobile communication method)
The mobile communication method according to the embodiment will be described below. FIG. 7 is a sequence diagram showing a mobile communication method according to the embodiment.

As shown in FIG. 7, in step S10, the UE 100 is in the RRC idle state.

In step S11, the UE 100 starts a random access procedure. Specifically, UE100 transmits the message (Random Access Preamble) comprised by the random access preamble selected from the some random access preamble to eNB200 (connection target cell). The UE 100 increases the transmission power of the message (Random Access Preamble) every time the message (Random Access Preamble) is transmitted.

Here, when the UE 100 has a function of selecting a selection target cell in the random access procedure, it is preferable to transmit a specific message (Random Access Preamble). The specific message may be a message (Random Access Preamble) composed of a specific preamble used for random access, or a message (Random Access Preamble) transmitted using a specific resource (resource block). May be.

On the other hand, when the UE 100 does not have a function of selecting a selection target cell in the random access procedure, it is preferable to transmit an existing message (Random Access Preamble) different from a specific message. The existing message may be a message (Random Access Preamble) configured by an existing preamble different from the specific preamble used for random access, and is transmitted using a resource (resource block) other than the specific resource. It may be a message (Random Access Preamble).

Thereby, the eNB 200 (connection target cell) determines whether the UE 100 has the function of selecting the selection target cell in the random access procedure according to the type of the random access preamble or the type of the resource used for transmitting the message. can do.

In step S12, the UE 100 receives a response message (Random Access Response) to the message (Random Access Preamble) from the eNB 200 (connection target cell).

Here, the response message (Random Access Response) is an example of a predetermined message that prompts the selection of the selection target cell used as the serving cell, as described above. The eNB 200 (connection target cell) transmits a response message (Random Access Response) as a predetermined message when the load of the connection target cell is equal to or higher than a certain level. Note that such a response message (Random Access Response) is that the eNB 200 (connection target cell) does not accept a message (RRC Connection Request) requesting the setting of an RRC connection, or an eRC 200 (connection target cell) receives an RRC layer message. ) May not be accepted.

It should be noted that the response message (Random Access Response) used as the predetermined message prompting the selection of the selection target cell used as the serving cell is distinguished from the response message (Random Access Response) for continuing the random access procedure. is there. For example, the two are distinguished by a 1-bit flag or the like.

In step S13, the UE 100 (control unit) suspends the random access procedure in accordance with a predetermined message received from the eNB 200 (connection target cell) in the random access procedure, and selects a selection target to be used as a serving cell from a plurality of cells. Select a cell. The information included in the predetermined message and the selection method of the selection target cell upon reception of the predetermined message are as described above.

(Action and effect)
The UE 100 (control unit) according to the embodiment interrupts the random access procedure and selects the selection target cell in response to a predetermined message received from the connection target cell in the random access procedure. That is, when the load of the connection target cell is high, the UE 100 is promptly prompted to select the selection target cell. Therefore, the load distribution of each cell can be appropriately performed. In other words, the UE 100 in the RRC idle state can be appropriately distributed to each cell.

[Modification 1]
Hereinafter, a first modification of the embodiment will be described. In the following, differences from the embodiment will be mainly described.

Specifically, in the embodiment, the predetermined message received from the connection target cell in the random access procedure includes recommendation information including a list of candidate cells that can be selected as the selection target cell. On the other hand, in the first modification, the UE 100 (control unit) notifies the connection target cell of an adjacent cell adjacent to the connection target cell as a candidate cell that can be selected as the selection target cell in the random access procedure.

Specifically, UE100 (control part) notifies a candidate cell to a connection object cell by performing a random access procedure using the random access preamble matched with the candidate cell.

Here, the random access preamble may be associated with one candidate cell. In such a case, the recommendation information may not include the candidate cell list and may include the candidate cell list including one candidate cell notified from the UE 100. The UE 100 (control unit) may omit the above-described evaluations (B1) to (B3) and use one candidate cell notified from the UE 100 to the connection target cell as a serving cell.

Alternatively, the random access preamble may be associated with a plurality of candidate cells. In such a case, the predetermined message includes information indicating a cell selected from the plurality of candidate cells notified from the UE 100. For example, the recommendation information may include a cell selected from a plurality of candidate cells notified from the UE 100. UE100 (control part) omits the evaluation shown to the above-mentioned (B1)-(B3), when the cell selected by the connection object cell is one cell, the cell selected by the connection object cell. May be used as the serving cell. When the cell selected by the connection target cell is a plurality of cells, the UE 100 (control unit) selects the selection target cell from the plurality of cells according to the evaluations described in (B1) to (B3) above. You may.

Here, when the random access preamble is associated with one candidate cell, the response message (Random Access Response), which is a predetermined message, includes a 1-bit flag indicating that the response message is the predetermined message. The 1-bit flag means that the random access procedure for the connection target cell is interrupted to instruct the selection of the selection target cell used as the serving cell. On the other hand, when the random access preamble is associated with a plurality of candidate cells, the response message (Random Access Response) that is a predetermined message is narrowed down by the eNB 200 (connection target cell) from among the plurality of candidate cells. It is preferable to include an index indicating the candidate cell.

[Modification 2]
Hereinafter, a second modification of the embodiment will be described. In the following, differences from the embodiment will be mainly described.

In Modification 2, the UE 100 (control unit) restarts the interrupted random access procedure for the selection target cell when the selection target cell is selected after suspending the random access procedure for the connection target cell. To do. In such a case, the connection target cell notifies the selection target cell of the RACH information (TA (Timing Advance), random access preamble, etc.) used in the random access procedure via the X2 interface. As a result, the random access procedure can be shortened as compared with the case where the random access procedure is restarted from the beginning for the selection target cell.

When the connection target cell has notified the UE 100 of the recommendation information including the list of candidate cells, it is preferable to notify the RACH information to all the cells included in the list of candidate cells. Alternatively, the connection target cell preferably notifies the RACH information to all the candidate cells notified from the UE 100 when the candidate cells are notified from the UE 100.

Note that the connection target cell and the selection target cell are preferably cells provided in the same eNB 200 (co-located or quasi-collocated).

(Mobile communication method)
The mobile communication method according to the second modification will be described below. 8 and 9 are sequence diagrams showing a mobile communication method according to the second modification. 8 and 9 exemplify a case where the connection target cell is the cell #1 and the selection target cell is the cell #2.

First, a case where the random access procedure for the selection target cell succeeds will be described with reference to FIG.

As shown in FIG. 8, in step S20, the UE 100 is in the RRC idle state, as in step S10.

In step S21, the UE 100 starts the random access procedure as in step S11. Specifically, UE100 transmits the message (Random Access Preamble) comprised by the random access preamble selected from the some random access preamble to eNB200 (cell #1).

In step S22, similarly to step S12, the UE 100 receives a response message (Random Access Response) to the message (Random Access Preamble) from the eNB 200 (cell #1). Here, the response message (Random Access Response) is an example of a predetermined message that prompts the selection of the selection target cell used as the serving cell, as described above.

In step S23, similarly to step S13, the UE 100 suspends the random access procedure according to a predetermined message received from the eNB 200 (cell #1) in the random access procedure, and uses it as a serving cell from a plurality of cells. Select the cell to be selected. Here, the cell #2 is selected as the selection target cell.

In step S24, the UE 100 transmits a message (Scheduled Tramsmission) including predetermined information to the eNB 200 (cell #1) at the timing determined by the reception timing of the response message (Random Access Response).

In step S25, the eNB 200 (cell #1) transmits a message (Connection routed) including the RACH information used in the random access procedure to the eNB 200 (cell #2).

In step S26, the eNB 200 (cell #2) permits the UE 100 to connect to the cell #2.

In step S27, the eNB 200 (cell #2) transmits a message (Connection routed ACK) indicating that the UE 100 is permitted to connect to the cell #2 to the eNB 200 (cell #1).

In step S28, the UE 100 receives a response message (Contention Resolution) to the message (Scheduled Tramsmission) from the eNB 200 (cell #2).

As described above, by restarting the random access procedure for the eNB 200 (cell #2), the random access procedure for the eNB 200 (cell #2) is successful.

Secondly, a case where the random access procedure for the selection target cell fails will be described with reference to FIG.

As shown in FIG. 9, in step S30, the UE 100 is in the RRC idle state, as in step S10.

In step S31, the UE 100 starts the random access procedure as in step S11. Specifically, UE100 transmits the message (Random Access Preamble) comprised by the random access preamble selected from the some random access preamble to eNB200 (cell #1).

In step S32, similarly to step S12, the UE 100 receives a response message (Random Access Response) to the message (Random Access Preamble) from the eNB 200 (cell #1). Here, the response message (Random Access Response) is an example of a predetermined message that prompts the selection of the selection target cell used as the serving cell, as described above.

In step S33, similarly to step S13, the UE 100 suspends the random access procedure according to a predetermined message received from the eNB 200 (cell #1) in the random access procedure, and uses it as a serving cell from among a plurality of cells Select the cell to be selected. Here, the cell #2 is selected as the selection target cell.

In step S34, the UE 100 transmits a message (Scheduled Tramsmission) including predetermined information to the eNB 200 (cell #1) at the timing determined by the reception timing of the response message (Random Access Response).

In step S35, the eNB 200 (cell #1) transmits a message (Connection routed) including the RACH information used in the random access procedure to the eNB 200 (cell #2).

In step S36, the eNB 200 (cell #2) rejects the connection of the UE 100 to the cell #2.

In step S37, the eNB 200 (cell #2) transmits a message (Connection routed NACK) indicating that the connection of the UE 100 to the cell #2 is rejected to the eNB 200 (cell #1).

In step S38, the UE 100 receives, from the eNB 200 (cell #1), a response message (Content Resolution) to the message (Scheduled Tramsmission).

As described above, the random access procedure for the eNB 200 (cell #2) is restarted, but the random access procedure for the eNB 200 (cell #2) fails.

8 and 9, in step S24 or step S34, the UE 100 transmits a message (Scheduled Tramsmission) including predetermined information to the eNB 200 (cell #1), but the second modification is not limited to this. Not a thing. Specifically, before step S24 and step S34, if the RACH information used in the random access procedure for the eNB200 (cell #1) is notified to the eNB200 (cell #2), the eNB200 (cell In step S22 or step S32, #1) may notify the UE 100 by a response message (Random Access Response) that the random access procedure for the eNB 200 (cell #2) can be continued. The UE 100 may send a message (Scheduled Tramsmission) including predetermined information to the eNB 200 (cell #2) in response to the response message (Random Access Response) received in step S22 or step S32.

Further, the RACH information used in the random access procedure involving the selection of the selection target cell may be exchanged between the eNB 200 (cell #1) and the eNB 200 (cell #2). Such exchange of RACH information may be performed before the UE 100 transmits the random access preamble to the eNB 200 (cell #1). In such a case, the eNB 200 (cell #1) does not have to notify the eNB 200 (cell #2) of the RACH information used in the random access procedure for the eNB 200 (cell #1).

[Modification 3]
Hereinafter, a modified example 3 of the embodiment will be described. In the following, differences from the embodiment will be mainly described.

Specifically, in the embodiment, a response message (Random Access Response) is used as the predetermined message prompting the selection of the selection target cell used as the serving cell. On the other hand, in the third modification, a message (Contention Resolution) for avoiding uplink transmission conflict in the random access procedure is used as the predetermined message for prompting the selection of the selection target cell used as the serving cell.

(Mobile communication method)
The mobile communication method according to Modification 3 will be described below. FIG. 10 is a sequence diagram showing a mobile communication method according to the third modification.

As shown in FIG. 10, in step S40, the UE 100 is in the RRC idle state.

In step S41, the UE 100 starts a random access procedure. Specifically, UE100 transmits the message (Random Access Preamble) comprised by the random access preamble selected from the some random access preamble to eNB200 (connection target cell). The UE 100 increases the transmission power of the message (Random Access Preamble) every time the message (Random Access Preamble) is transmitted.

In step S42, the UE 100 receives a response message (Random Access Response) to the message (Random Access Preamble) from the eNB 200 (connection target cell).

In step S43, the UE 100 transmits a message (Scheduled Tramsmission) including predetermined information at a timing determined by the reception timing of the response message (Random Access Response).

In step S44, the UE 100 receives a response message (Contention Resolution) to the message (Scheduled Tramsmission) from the eNB 200 (connection target cell).

Here, the response message (Contention Resolution) is an example of a predetermined message that prompts the selection of the selection target cell used as the serving cell, as described above. The eNB 200 (connection target cell) transmits a response message (Contention Resolution) as a predetermined message when the load of the connection target cell is equal to or higher than a certain level.

It should be noted that the response message (Contention Resolution) used as the predetermined message for prompting the selection of the selection target cell used as the serving cell is distinguished from the response message (Contention Resolution) for continuing the random access procedure. For example, the two are distinguished by a 1-bit flag or the like.

In step S45, the UE 100 (control unit) suspends the random access procedure according to a predetermined message received from the eNB 200 (connection target cell) in the random access procedure, and selects a serving cell to be used as a serving cell from a plurality of cells. Select a cell. The information included in the predetermined message and the selection method of the selection target cell upon reception of the predetermined message are as described above.

[Other Embodiments]
Although the present invention has been described by the embodiments described above, it should not be understood that the description and drawings forming a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples, and operation techniques will be apparent to those skilled in the art.

Although not particularly touched in the embodiment, the connection target cell, while activating the timer by the transmission of a predetermined message including the recommendation information, without restarting the random access procedure, when the timer value reaches a predetermined threshold value, It may be determined that the serving cell of the UE 100 has switched to another cell.

In the embodiment, the case where the predetermined message is Random Access Response or Contention Resolution has been illustrated. However, the embodiment is not limited to this. Specifically, the predetermined message may be a new message defined by extension of the random access procedure. For example, a 1-bit flag indicating whether or not it is recommended to select a cell other than the connection target cell as the selection target cell may be added to the Random Access Response or the Contention Resolution. In addition, a new message such as a downlink message including a list of candidate cells that can be selected as a selection target cell and scheduling information of the downlink message may be introduced. The scheduling information of the downlink message may be information included in Random Access Response or Contention Resolution (for example, RAR scheduling information). Here, the downlink message scheduled by the scheduling information may be transmitted in a subframe in which the scheduling information is transmitted, and is transmitted in a subframe after a predetermined number of subframes from the subframe in which the scheduling information is transmitted. May be. The interval between the subframe in which the scheduling information is transmitted and the subframe in which the downlink message is transmitted (that is, the above-described predetermined number) may be a predetermined value or a value included in the scheduling information. May be.

In the embodiment, the parameter used in the selection of the selection target cell is included in the predetermined message. However, the embodiment is not limited to this. The parameter used in the selection of the selection target cell may be included in the broadcast information (SIB; System Information Block) broadcast from the eNB 200 (connection target cell).

In the embodiment, the recommendation information is included in the predetermined message. However, the embodiment is not limited to this. The recommendation information may be included in the broadcast information (SIB: System Information Block) broadcast from the eNB 200 (connection target cell).

Although not particularly mentioned in the embodiment, as shown in the second modification, in the case where the random access procedure is restarted for a cell other than the connection target cell, the other cell is connected to the other cell and the UE 100. When is completed, the connection completion may be notified to the connection target cell.

In the second modification, the procedure for restarting the interrupted random access procedure when the connection target cell and the selection target cell are cells provided in the same eNB 200 has been described. However, when the connection target cell and the selection target cell are not provided in the same eNB 200, the random access procedure may be redone from the beginning for the selection target cell.

Although not particularly mentioned in the embodiment, a program that causes a computer to execute each process performed by the UE 100 and the eNB 200 may be provided. Further, the program may be recorded in a computer-readable medium. A computer readable medium can be used to install the program on a computer. Here, the computer-readable medium in which the program is recorded may be a non-transitory recording medium. The non-transitory recording medium is not particularly limited, but may be a recording medium such as a CD-ROM or a DVD-ROM.

Alternatively, a chip configured by a memory that stores a program for executing each process performed by the UE 100 and the eNB 200 and a processor that executes the program stored in the memory may be provided.

In the embodiment, the LTE system has been described as an example of the mobile communication system. However, the embodiment is not limited to this. The mobile communication system may be a system other than the LTE system.

[Cross reference]
The entire contents of Japanese Patent Application No. 2015-080774 (April 10, 2015) are incorporated herein by reference.

The present invention is useful in the communication field.

Claims (7)

From a connection target cell in a random access procedure, a receiving unit for receiving a random access response message including recommendation information recommending that a cell other than the connection target cell be used as a serving cell to be selected ,
In response to reception of the random access response message including the recommendation information, along with interrupting the random access procedure, based on the recommendation information, from among a plurality of cells, and a control unit for selecting the selection target cell, Equipped with
The control unit restarts the interrupted random access procedure by transmitting a message (Scheduled Tramsmission) including predetermined information to the selection target cell,
The connection target cell and the selected target cell, the user terminal characterized by Rukoto provided in the same base station. The user terminal according to claim 1, wherein the random access response message including the recommendation information is transmitted from the connection target cell when the load of the connection target cell is a certain amount or more. The user terminal according to claim 1 , wherein the recommendation information includes a list of candidate cells that can be selected as the selection target cell. The recommendation information includes an additional offset to be applied to a metric for determining whether or not the candidate cell should be selected as the selection target cell so that the candidate cell is likely to be selected as the selection target cell. the user terminal according to 請 Motomeko 3 characterized. The control unit, in the case of applying the additional offset, whether or not the metric for determining whether or not to select the connection target cell as the selection target cell should select the candidate cell as the selection target cell The user terminal according to claim 4 , wherein, when the metric for judging is below, a selection of the selection target cell is started. The control unit, in the case of applying the additional offset, whether or not the metric for determining whether or not to select the connection target cell as the selection target cell should select the candidate cell as the selection target cell The user terminal according to claim 4 , wherein the random access procedure is restarted when the metric for judging is exceeded. User terminal, from the connection target cell in the random access procedure, a step of receiving a random access response message including recommendation information recommending to select a selection target cell using a cell other than the connection target cell as a serving cell ,
Said user terminal, in response to reception of the random access response message including the recommendation information, along with interrupting the random access procedure, based on the recommendation information, from among a plurality of cells, selecting the selected target cell Steps to
A step in which the user terminal restarts the interrupted random access procedure by transmitting a message (Scheduled Tramsmission) including predetermined information with respect to the selection target cell to the selection target cell; Prepare ,
The connection target cell and the selected target cell, the mobile communication method comprising Rukoto provided in the same base station.

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